1. Field of the Invention
The present invention relates to an ion current detector, and more particularly, to an ion current detector for detecting an ion current generated in an internal combustion engine.
2. Description of the Related Art
In a conventional ion current detector, a high voltage is applied to a discharge gap between a center electrode and a ground electrode of a spark plug to generate a spark discharge. When an ion current flowing through the discharge gap is to be detected, a connection terminal portion of the spark plug is coated with a conductive coating such as a coating plated with gold or silver to ensure the electrical conductivity of the connection terminal portion (see, for example, JP 3605962 B).
As described in JP 3605962 B, the structure in which the connection terminal portion is coated with the conductive coating such as the coating plated with gold or silver to ensure the electrical conductivity of the connection terminal portion has such a problem that a manufacturing cost increases.
There is also the following problem. When the spark plug smolders, soot generates between the electrodes, which causes a leak current to flow. When a pseudo state occurs, in which the ion current continuously flows (hereinafter, referred to as smoldering state), a smoldering current increases at the time of supplying an ignition signal because of an ignition signal turning-ON +V2. Therefore, an ion current detection signal to be transmitted to an engine control unit (ECU) exceeds a dynamic range of the ECU, with the result that it is difficult to transmit an accurate ion current detection signal to the ECU.